First, some random brain spewing:
Monte Cook's Book of Experimental Might revised the spell list to be a 20 level progression, and I admit that I really liked it.
On the other hand, that level of granularity has it's own difficulties. It becomes more difficult to remember what level different spells are, and you end up with a 20 levels worth of spell slots to choose.
On the third hand, 20 levels of spells means that there are fewer spells at each level, making potentially easier to choose your daily allotment of spells.
Now, a consideration of the 3E spell levels:
In third edition, those first three spell levels really seemed to matter. First level spells felt constrained. Second level spells started to show glimmers of what a wizard could do. Third level spells are when the wizard really opened up.
After third level, spell level stop carrying much weight. What's the difference between a sixth and a seventh level spell? It all sort of blurs.
Level 9, of course, has the capstone powers: Disjunction, Meteor Swarm, Power Word Kill, and others. There's definition there. So, really, it is levels four through eight that lack a strong sense of identity.
I would strongly consider doing some consolidating at this point. Merge levels four and five, six and seven, and eight and nine to leave behind a six level spell progression. Then replace linear spell level progression with something that slows down as a character levels. Fill in the gaps with interesting class features.
For example:
1 1st level spells
2 2nd level spells
3
4 3rd level spells
5
6 Class feature
7 4th level spells
8
9 Class feature
10
11 5th level spells
12
13 Class feature
14
15 Class feature
16 6th level spells
17
18 Class feature
19
20 Class feature
This sequence shows the distance between spell levels growing numerically (+1, +2, +3, +4, +5). Then the gaps were filled so that the wizard never went more than two levels without receiving either new spell levels or a class feature. Unexpectedly, the final ability landed on level 20, which would be perfect for a capstone ability.
Overall, I won't say this is my preference, just that I would like to see it tested.